1. Field of the Invention
This invention relates to a magnetic recording medium, which comprises a magnetic layer constituted of a thin ferromagnetic metal film, and a process for producing the magnetic recording medium.
2. Description of the Prior Art
Ordinarily, magnetic recording media, such as magnetic tapes, floppy disks, and hard disks, have heretofore been produced with processes for forming a magnetic layer, a protective layer, and the like, on a non-magnetic substrate. As such magnetic recording media, magnetic recording media provided with a thin ferromagnetic metal film, which is formed with a vacuum film forming process, such as a sputtering process or a vacuum evaporation process, as the magnetic layer have widely been used in practice. The magnetic recording media provided with the magnetic layer, which is formed with the sputtering process or the vacuum evaporation process, have characteristics such that a high level of magnetic energy can be obtained easily. Also, the magnetic layer having a high level of surface smoothness can be obtained easily by rendering the surface of the non-magnetic substrate smooth. Therefore, the spacing loss can be kept small, and good electromagnetic characteristics can be obtained. Accordingly, the magnetic recording media provided with the magnetic layer, which is formed with the sputtering process or the vacuum evaporation process, are suitable as materials for recording magnetic information at high densities. In particular, the sputtering process is advantageous over the vacuum evaporation process in that the magnetic energy can be enhanced even further. Therefore, the sputtering process is employed for the production of magnetic recording media, such as hard disks, on which the magnetic information is to be recorded at high densities.
Nowadays, there is a strong demand for magnetic recording media on which the magnetic information can be recorded at high densities, and electromagnetic characteristics better than those in the past are required. Therefore, as for the magnetic recording media, such as magnetic tapes or floppy disks, in which the flexible polymer film, such as a polyethylene terephthalate film or a polyethylene naphthalate film, is employed as the non-magnetic substrate, such that, for example, the recording density may be enhanced, the magnetic layer constituted of a thin ferromagnetic metal film should preferably be formed with the sputtering process or the vacuum evaporation process.
However, the heat resistance of the polymer film is low. Therefore, in cases where the magnetic layer is formed on the polymer film with the sputtering process, or in cases where the magnetic layer is formed on the polymer film with the vacuum evaporation process at a high rate of vapor deposition, the problems occur in that the polymer film or its surface becomes heated and the non-magnetic substrate becomes deformed. Also, the problems occur in that the surface of the non-magnetic substrate is deteriorated due to separating of oligomers, or the like. As a result, it becomes difficult to obtain good surface smoothness of the non-magnetic substrate or the magnetic layer.
In order to solve the problems, the techniques described below have been proposed.
In one of the proposed techniques, a heat-resistant resin is utilized as the material for the polymer film serving as the non-magnetic substrate. It may be considered that a polyimide resin, or the like, may be employed as the heat-resistant resin. However, in general, polyimide resin films are expensive. Also, for technical reasons, it is difficult to prepare and use polyimide films having very smooth surfaces and good surface characteristics. Therefore, the proposed technique is not suitable for use in practice.
In different proposed techniques, a prime-coating layer is formed on, for example, a comparatively cheap polymer film, which is ordinarily used for the conventional magnetic recording media provided with a magnetic layer formed with a coating process, and the surface smoothness and the heat resistance of the film is thereby enhanced.
For example, in Japanese Unexamined Patent Publication No. 6(1994)-349042, a method for preparing a film having an appropriate level of surface characteristics is disclosed, wherein a resin layer containing fine grains is formed on a polymer film having a comparatively rough surface. However, in cases where ordinary resin binders employed in the disclosed method are used, if a magnetic layer is formed on the film with the sputtering process, the surface characteristics will be deteriorated markedly due to thermal damage.
Also, a method, wherein a polyethylene naphthalate is coated on a polyethylene terephthalate, and wherein the separating of oligomers due to heat is thereby restricted, is disclosed in, for example, Japanese Unexamined Patent Publication No. 7(1995)-225934. However, with the disclosed method wherein a polyethylene naphthalate is utilized, if the film is heated to a temperature of 200.degree. C., which is ordinarily set in the sputtering process, the surface characteristics will be deteriorated due to the separating of oligomers.
Further, a method, wherein a polyamide resin or a polyimide resin having a high heat resistance is coated onto a polymer film, is disclosed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-208717. In cases where such a material is used, a heat resistance capable of overcoming the temperature conditions in the sputtering process can be imparted to the polymer film. However, the polyamide resins and the polyimide resins have a low solubility in general-purpose solvents, and therefore solvents, which are difficult to process, must be used for such resins. Even if the resins were soluble in general-purpose solvents, since the viscosity of the solutions is high, a thin and uniform coating layer cannot be obtained, and it would be difficult to enhance the surface characteristics of the polymer film. Also, with the disclosed method, it is difficult to dry the solvent sufficiently, and the amount of the solvent remaining in the coating layer cannot be kept small. Therefore, the blocking readily occurs, and thus the coating layer, which has been formed on the front surface of the polymer film, and the back surface of the polymer film adhere to each other when the polymer film is wound up. Further, there is the risk that, when the magnetic layer is formed on the coating layer in a vacuum tank, the residual solvent volatilizes from the coating layer and contaminates the region in the vacuum tank.
In order for a coating layer having a high heat resistance to be obtained, it is efficient to form a coating layer of an inorganic material. For example, it may be considered to form a layer of silica, which is obtained from hydrolysis of a silane compound, or a layer of a metal oxide, which is obtained from a metal alkoxide. However, such a layer of the inorganic material cannot follow a thermal change of the non-magnetic substrate. As a result, cracks occur on the surface of the layer of the inorganic material, and the magnetic layer formed on the layer of the inorganic material also becomes cracked.
As a magnetic recording medium capable of solving the aforesaid problems to some extent, the applicant proposed a magnetic recording medium provided with a prime-coating layer, which is principally constituted of Si--O or Si--O--N, in Japanese Unexamined Patent Publication No. 8(1996)-329443. The proposed magnetic recording medium is provided with the prime-coating layer, which has smooth surface characteristics and is free from deterioration of surface characteristics and the occurrence of cracking even during the formation of a magnetic layer thereon with a sputtering process accompanied by heating of the substrate. Also, the prime-coating layer of the proposed magnetic recording medium causes no blocking to occur. However, when a coating composition for forming the prime-coating layer of the proposed magnetic recording medium is applied onto the non-magnetic substrate, run-away of the coating composition often occurs. Therefore, it was difficult to form a uniform prime-coating layer. Further, a wet-on-wet coating technique cannot be employed for the prime-coating layer of the proposed magnetic recording medium, and therefore the thickness of the prime-coating layer cannot be set to be large.
As described above, there has heretofore been proposed no prime-coating layer, which has smooth surface characteristics, which is free from deterioration of surface characteristics and the occurrence of cracking even during the formation of a magnetic layer thereon with a sputtering process accompanied by heating of the substrate, which does not cause blocking to occur, and which is uniform and is capable of being subjected to the wet-on-wet coating technique.